1. Field of the Invention
The present invention relates to a device for detecting a pneumatic pressure drop of tires of a four-wheel vehicle in order to give an alarm.
2. Related Background Art
As a safety device for a four-wheel vehicles such as automobiles and trucks, tire pressure drop detecting devices have recently been proposed, and some of them have been put to practical use.
A pneumatic pressure drop alarm device has been developed because its importance is recognized for the reason shown below. That is, when the pneumatic pressure is low, the temperature of a tire increases due to an increase of tire deflection. Therefore, the strength of the polymer material used for the tire is lowered and the tire is more likely to burst. However, even if the tire is deflated, the driver is often unaware of the deflation.
As a method of detecting a pneumatic pressure drop by the pneumatic pressure drop alarm device, for example, there is a method of utilizing a difference in respective rotational angular velocities F.sub.1, F.sub.2, F.sub.3 and F.sub.4 (referred to as a "rotational angular velocity F.sub.i " hereinafter) of four tires W.sub.1, W.sub.2, W.sub.3 and W.sub.4 of the vehicle. (The tires W.sub.1 and W.sub.2 correspond to left and right front tires, and the tires W.sub.3 and W.sub.4 correspond to left and right rear tires, respectively. In addition, these are referred to as a "tire W.sub.i " hereinafter.)
According to this method, the rotational angular velocity F.sub.i of the tire W.sub.i is detected every predetermined sampling period .DELTA.T, based on a signal to be generated by a wheel speed sensor mounted on the tire W.sub.i. When the dynamic load radiuses (which mean apparent rolling radiuses of the respective tires calculated by dividing by 2.pi. a travel distance of the vehicle while the tires make one free revolution) of the tires W.sub.i are the same, at the same time that the vehicle is traveling linearly, the rotational angular velocities F.sub.i are the same.
However, the dynamic load radius of the tire W.sub.i varies depending on the change in pneumatic pressure of the tire W.sub.i. When the pneumatic pressure of the tire W.sub.i drops, the dynamic load radius becomes smaller than that at a normal internal pressure. Accordingly, the rotational angular velocity F.sub.i of the tire W.sub.i becomes higher than that at a normal internal pressure, thereby making it possible to detect the pneumatic pressure drop of the tire W.sub.i, based on the difference in the rotational angular velocities F.sub.i. A judging equation for detecting the pneumatic pressure drop of the tire W.sub.i is shown below (see Japanese Laid-Open Patent Publication Nos. 63-305011 and 4-212609). EQU D=2[(F.sub.1 +F.sub.4)-(F.sub.2 +F.sub.3)]/(F.sub.1 +F.sub.2 +F.sub.3 +F.sub.4) (1)
For example, if it is assumed that the dynamic load radiuses of the respective tires W.sub.i are the same, the rotational angular velocities F.sub.i are the same (F.sub.1 =F.sub.2 =F.sub.3 =F.sub.4). Accordingly, the judging value D becomes 0. Then, threshold values D.sub.TH1 and D.sub.TH2 are set (D.sub.TH1, D.sub.TH2 &gt;0). When the following judging expression: EQU D&lt;-D.sub.TH1 or D&gt;D.sub.TH2
is satisfied, it is judged that the tire W.sub.i having a pneumatic pressure drop is present. When the judging expression is not satisfied, it is judged that the tire W.sub.i having a pneumatic pressure drop is not present. When it is judged that a tire W.sub.i has a pneumatic pressure drop, an alarm is given by using a display or a speaker.
In the pneumatic pressure drop detection processing at the time of actual traveling, when front/rear accelerations and left/right accelerations of the vehicle are large, for example, when the load transfers around the vehicle at the time of traveling with rapid acceleration or traveling around a curve, the driving tires are sometimes locked or idled, that is, the wheel is not rolling due to slippage or some other cause. Since the locking or idling does not always arise uniformly in left/right driving tires, revolution counts of the respective driving tires are different. As a result, D does not become equal to 0, thereby causing a wrong judgment of reduced pressure. Therefore, for example, there is disclosed a technique of removing the data corresponding to the rotational angular velocity F.sub.i in Japanese Utility Model Publication No. 4-11844.
In the technique as disclosed in Japanese Utility Model Publication No. 4-11844, when it is considered that the tire W.sub.i is in the idling or locking state, the rotational angular velocity F.sub.i to be calculated at that point is ignored in detecting whether the pneumatic pressure of the tire W.sub.i drops or not. More specifically, when it is judged that both ratios F.sub.1 /F.sub.2 and F.sub.3 /F.sub.4 of the rotational angular velocities F.sub.1, F.sub.2, F.sub.3 and F.sub.4 of the left and right front tires W.sub.1 and W.sub.2 as well as left and right rear tires W.sub.3 and W.sub.4 are not within a predetermined range, it is considered that the tire W.sub.i is in the state of one-wheel idling or locking, and the data of the rotational angular velocity F.sub.i is ignored.
In the above tire pressure drop detection technique, the rotational angular velocity F.sub.i calculated when both ratios F.sub.1 /F.sub.2 and F.sub.3 /F.sub.4 are not within the predetermined range is merely ignored. Therefore, as shown in FIG. 15, if it is judged only by the ratio F.sub.1 /F.sub.2 or F.sub.3 /F.sub.4 when the vehicle is traveling while one side thereof is running on an irregular road surface (e.g. a shoulder of a non-paved road such as grassy or sandy place, or a road side covered with snow) ("one-sided irregular road") because of a car on the opposite lane, causing idling little by little because the irregular road side is slippery, there is a possibility of a wrong judging value, since the above condition is not normally satisfied, at the time of traveling on the one-sided irregular road. Accordingly, there is a disadvantage in that the traveling data which is not suitable for the reduced pressure judgment are adopted, thereby giving a false alarm.
Consequently, it is impossible to judge accurately only by judging whether the left/right wheel ratios F.sub.1 /F.sub.2 and F.sub.3 /F.sub.4 are within the predetermined range or not, as disclosed in Japanese Utility Model Publication No. 4-11844.